VM (Executing JavaScript)

You can access this module with:

const vm = require('vm');

JavaScript code can be compiled and run immediately or compiled, saved, and run later.

Class: Script

A class for holding precompiled scripts, and running them in specific sandboxes.

new vm.Script(code, options)

Creating a new Script compiles code but does not run it. Instead, the created vm.Script object represents this compiled code. This script can be run later many times using methods below. The returned script is not bound to any global object. It is bound before each run, just for that run.

The options when creating a script are:

• filename: allows you to control the filename that shows up in any stack traces produced from this script.
• lineOffset: allows you to add an offset to the line number that is displayed in stack traces
• columnOffset: allows you to add an offset to the column number that is displayed in stack traces
• displayErrors: whether or not to print any errors to stderr, with the line of code that caused them highlighted, before throwing an exception. Applies only to syntax errors compiling the code; errors while running the code are controlled by the options to the script's methods.
• timeout: a number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

script.runInContext(contextifiedSandbox[, options])

Similar to vm.runInContext() but a method of a precompiled Script object. script.runInContext() runs script's compiled code in contextifiedSandbox and returns the result. Running code does not have access to local scope.

script.runInContext() takes the same options as script.runInThisContext().

Example: compile code that increments a global variable and sets one, then execute the code multiple times. These globals are contained in the sandbox.

const util = require('util');
const vm = require('vm');

var sandbox = {
  animal: 'cat',
  count: 2
};

var context = new vm.createContext(sandbox);
var script = new vm.Script('count += 1; name = "kitty"');

for (var i = 0; i < 10; ++i) {
  script.runInContext(context);
}

console.log(util.inspect(sandbox));

// { animal: 'cat', count: 12, name: 'kitty' }

Note that running untrusted code is a tricky business requiring great care. script.runInContext() is quite useful, but safely running untrusted code requires a separate process.

script.runInNewContext([sandbox][, options])

Similar to vm.runInNewContext() but a method of a precompiled Script object. script.runInNewContext() contextifies sandbox if passed or creates a new contextified sandbox if it's omitted, and then runs script's compiled code with the sandbox as the global object and returns the result. Running code does not have access to local scope.

script.runInNewContext() takes the same options as script.runInThisContext().

Example: compile code that sets a global variable, then execute the code multiple times in different contexts. These globals are set on and contained in the sandboxes.

const util = require('util');
const vm = require('vm');

const sandboxes = [{}, {}, {}];

const script = new vm.Script('globalVar = "set"');

sandboxes.forEach((sandbox) => {
  script.runInNewContext(sandbox);
});

console.log(util.inspect(sandboxes));

// [{ globalVar: 'set' }, { globalVar: 'set' }, { globalVar: 'set' }]

Note that running untrusted code is a tricky business requiring great care. script.runInNewContext() is quite useful, but safely running untrusted code requires a separate process.

script.runInThisContext([options])

Similar to vm.runInThisContext() but a method of a precompiled Script object. script.runInThisContext() runs script's compiled code and returns the result. Running code does not have access to local scope, but does have access to the current global object.

Example of using script.runInThisContext() to compile code once and run it multiple times:

const vm = require('vm');

global.globalVar = 0;

const script = new vm.Script('globalVar += 1', { filename: 'myfile.vm' });

for (var i = 0; i < 1000; ++i) {
  script.runInThisContext();
}

console.log(globalVar);

// 1000

The options for running a script are:

• filename: allows you to control the filename that shows up in any stack traces produced.
• lineOffset: allows you to add an offset to the line number that is displayed in stack traces
• columnOffset: allows you to add an offset to the column number that is displayed in stack traces
• displayErrors: whether or not to print any errors to stderr, with the line of code that caused them highlighted, before throwing an exception. Applies only to runtime errors executing the code; it is impossible to create a Script instance with syntax errors, as the constructor will throw.
• timeout: a number of milliseconds to execute the script before terminating execution. If execution is terminated, an Error will be thrown.

vm.createContext([sandbox])

If given a sandbox object, will "contextify" that sandbox so that it can be used in calls to vm.runInContext() or script.runInContext(). Inside scripts run as such, sandbox will be the global object, retaining all its existing properties but also having the built-in objects and functions any standard global object has. Outside of scripts run by the vm module, sandbox will be unchanged.

If not given a sandbox object, returns a new, empty contextified sandbox object you can use.

This function is useful for creating a sandbox that can be used to run multiple scripts, e.g. if you were emulating a web browser it could be used to create a single sandbox representing a window's global object, then run all <script> tags together inside that sandbox.

vm.isContext(sandbox)

Returns whether or not a sandbox object has been contextified by calling vm.createContext() on it.

vm.runInContext(code, contextifiedSandbox[, options])

vm.runInContext() compiles code, then runs it in contextifiedSandbox and returns the result. Running code does not have access to local scope. The contextifiedSandbox object must have been previously contextified via vm.createContext(); it will be used as the global object for code.

vm.runInContext() takes the same options as vm.runInThisContext().

Example: compile and execute different scripts in a single existing context.

const util = require('util');
const vm = require('vm');

const sandbox = { globalVar: 1 };
vm.createContext(sandbox);

for (var i = 0; i < 10; ++i) {
    vm.runInContext('globalVar *= 2;', sandbox);
}
console.log(util.inspect(sandbox));

// { globalVar: 1024 }

Note that running untrusted code is a tricky business requiring great care. vm.runInContext() is quite useful, but safely running untrusted code requires a separate process.

vm.runInDebugContext(code)

vm.runInDebugContext() compiles and executes code inside the V8 debug context. The primary use case is to get access to the V8 debug object:

const vm = require('vm');
const Debug = vm.runInDebugContext('Debug');
console.log(Debug.findScript(process.emit).name);  // 'events.js'
console.log(Debug.findScript(process.exit).name);  // 'internal/process.js'

Note that the debug context and object are intrinsically tied to V8's debugger implementation and may change (or even get removed) without prior warning.

The debug object can also be exposed with the --expose_debug_as= switch.

vm.runInNewContext(code[, sandbox][, options])

vm.runInNewContext() compiles code, contextifies sandbox if passed or creates a new contextified sandbox if it's omitted, and then runs the code with the sandbox as the global object and returns the result.

vm.runInNewContext() takes the same options as vm.runInThisContext().

Example: compile and execute code that increments a global variable and sets a new one. These globals are contained in the sandbox.

const util = require('util');
const vm = require('vm');

const sandbox = {
  animal: 'cat',
  count: 2
};

vm.runInNewContext('count += 1; name = "kitty"', sandbox);
console.log(util.inspect(sandbox));

// { animal: 'cat', count: 3, name: 'kitty' }

Note that running untrusted code is a tricky business requiring great care. vm.runInNewContext() is quite useful, but safely running untrusted code requires a separate process.

vm.runInThisContext(code[, options])

vm.runInThisContext() compiles code, runs it and returns the result. Running code does not have access to local scope, but does have access to the current global object.

Example of using vm.runInThisContext() and eval() to run the same code:

const vm = require('vm');
var localVar = 'initial value';

const vmResult = vm.runInThisContext('localVar = "vm";');
console.log('vmResult:', vmResult);
console.log('localVar:', localVar);

const evalResult = eval('localVar = "eval";');
console.log('evalResult:', evalResult);
console.log('localVar:', localVar);

// vmResult: 'vm', localVar: 'initial value'
// evalResult: 'eval', localVar: 'eval'

vm.runInThisContext() does not have access to the local scope, so localVar is unchanged. eval() does have access to the local scope, so localVar is changed.

In this way vm.runInThisContext() is much like an indirect eval() call, e.g. (0,eval)('code'). However, it also has the following additional options:

• filename: allows you to control the filename that shows up in any stack traces produced.
• lineOffset: allows you to add an offset to the line number that is displayed in stack traces
• columnOffset: allows you to add an offset to the column number that is displayed in stack traces
• displayErrors: whether or not to print any errors to stderr, with the line of code that caused them highlighted, before throwing an exception. Will capture both syntax errors from compiling code and runtime errors thrown by executing the compiled code. Defaults to true.
• timeout: a number of milliseconds to execute code before terminating execution. If execution is terminated, an Error will be thrown.

Example: Run a Server within a VM

The context of .runInThisContext() refers to the V8 context. The code passed to this VM context will have it's own isolated scope. To run a simple web server using the http module, for instance, the code passed to the context must either call require('http') on its own, or have a reference to the http module passed to it. For instance:

'use strict';
const vm = require('vm');

let code =
`(function(require) {

   const http = require('http');

   http.createServer( (request, response) => {
     response.writeHead(200, {'Content-Type': 'text/plain'});
     response.end('Hello World\\n');
   }).listen(8124);

   console.log('Server running at http://127.0.0.1:8124/');
 })`;

 vm.runInThisContext(code)(require);

Note: require() in the above case shares the state with context it is passed from. This might introduce risks when unknown code is executed, e.g. altering objects from the calling thread's context in unwanted ways. It is advisable to run vm code in a separate process.